It is known in the art to use a gasification process to convert a carbonaceous material into carbon monoxide and hydrogen. Generally a conversion is achieved by exposing a raw carbonaceous material at high temperature, often in excess of 850° C., with a controlled feed of oxygen or steam. A resultant gas that is produced is known as a synthesis gas or syngas. The syngas is a combustible energy source and can be more effectively and efficiently utilised in subsequent operations than a direct combustion of the original raw carbonaceous material itself.
Similarly, it is know to use a pyrolysis process to induce a chemical decomposition of a carbonaceous material to form a char and a syngas. Generally the pyrolysis process is similar to a gasification process though it occurs at a lower temperature (upward of 750° C.) and produces a solid residue rich in carbon content in addition to the syngas.
The carbonaceous material used for fuel in the abovementioned processes is often a biomass fuel. Biomass is generally biological matter derived from living organisms of both plant and animal variety. Biomass fuel is specifically carbon based and composed of organic molecules containing hydrogen. Biomass may include such organic material as garbage, waste (including plastics waste) and wood. Fossil fuels such as coal and oil are not generally included under the blanket name of biomass.
As mentioned above the gasification/pyrolysis process results in a gas mixture (syngas) containing various amounts of carbon monoxide and hydrogen. However, with certain fuels it is a problem that the syngas produced is rich in heavy long-chained carbon molecules or tar. Because of their large molecular weight the long-chained carbon molecules have a relatively low dew point and in practice these larger molecules tend to form tar deposits at cold spots in the pipework or in other gas cleaning apparatus or equipment. Such tar deposits can be a hindrance to the process by causing full or partial blockages leading to a consequent over pressurization and a decreased system performance, increased maintenance costs or significant equipment damage.
It is known in the art to use a scrubber to condense a tar deposit in order to form tar balls which can then be removed mechanically. It is also known to use oxygen injection to remove deposits in pipework however this method is not always suitable for subsequent direct injection of the syngas into, for example, a gas turbine. It is known to use catalysts which have proven to greatly reduce the formation of tar deposits however catalyst handling and regeneration all add significant costs to the process. Catalysts also increase head load and tube abrasion. Further process stages and costs are incurred at the end of the process in order to separate the catalyst from the char.
Using these methods the long-chained carbon molecules can be removed from the syngas, however the calorific value of the resultant syngas is correspondingly reduced.